Various mechanisms may contribute to the antiatherogenic potential of apolipoprotein A-I (apo A-I) and high density lipoproteins (HDLs). Therefore, the effect of adenovirus-mediated human apo A-I gene transfer or human apo A-I transgenesis on platelet-activating factor acetylhydrolase (PAF-AH) and arylesterase/paraoxonase (PON1) was studied in C57BL/6 and C57BL/6 apo E(-/-) mice. Human apo A-I transgenesis in C57BL/6 mice resulted in a 4.2-fold (P<0.0001) increase of PAF-AH and a 1.7-fold (P=0.0012) increase of PON1 activity. The apo E deficiency was associated with a 1.6-fold (P=0.008) lower PAF-AH and a 2.0-fold (P=0.012) lower PON1 activity. Human apo A-I transgenesis in C57BL/6 apo E(-/-)mice increased PAF-AH and PON1 activity by 2.1-fold (P=0.01) and 2.5-fold (P=0.029), respectively. After adenovirus-mediated gene transfer of human apo A-I into C57BL/6 apo E(-/-)mice, a strong correlation between human apo A-I plasma levels and PAF-AH activity was observed at day 6 (r=0.92, P<0.0001). However, PON1 activity failed to increase, probably as a result of cytokine-mediated inhibition of PON 1 expression. In conclusion, this study indicates that overexpression of human apo A-I increases HDL-associated PAF-AH activity. PON1 activity was also increased in human apo A-I transgenic mice, but not after human apo A-I gene transfer, a result that was probably related to cytokine production induced in the liver by the adenoviral vectors. Increased levels of these HDL-associated enzymes may contribute to the anti-inflammatory and antioxidative potential of HDL and thereby to the protection conferred by HDL against atherothrombosis.